Broke is waste or off-specification paper which is to be recycled. It is generally more desirable to recycle the broke by a process called repulping than it is to merely dispose of the broke as waste paper.
In the repulping process, the cellulose fibers which make up the broke are separated from the broke to make them usable for manufacturing paper. In the process, wet strength resin (such as a polyamide-epichlorhydrin resin) used to enhance the strength of the paper must be broken down since it binds the cellulose fibers together, forming a water impervious coating which impedes the repulping process goal of separating the cellulose fibers. Representative paper products containing wet strength resins are paper towels, tissues, and food wrappings. Typically, paper having wet strength resins will retain at least 15% of the paper's dry strength when wet. Paper without wet strength resin generally retains only 2-7% of its dry strength when wet.
Oxidation facilitates the break down of the wet strength resin to permit separation of the cellulose fibers. Traditionally, hypochlorite, particularly sodium hypochlorite, has been used by paper mills as the oxidizing agent, and has generally been used within a narrow, carefully maintained pH range and within a temperature range of from about 122.degree. F. (50.degree. C.) to 151.degree. F. (66.degree. C.). After broke has been successfully repulped, an agent is added to neutralize the remaining chlorine.
Environmental issues have been raised concerning the use of hypochlorite for repulping. These concerns range from the formation of suspected carcinogenic organic halides which are adsorbed by the pulp, to the problem of chloroform emission, and to the problem of adding toxic chlorinated hydrocarbons to the effluent stream. For these reasons, non-halogen containing compounds such as persulfates have been used to oxidize wet strength resin during the repulping process.
When recycling broke, sometimes the broke must be decolorized. This is achieved by decomposing the colorants used to color the broke. Colorants are chemicals that interact with light, causing the absorption of selective wavelengths in the visible spectrum (400 to 700 nm). Wave-lengths that are reflected (not absorbed) yield colors that are perceptible to the human eye. Colorants include both pigments and dyes and can be added during the paper making process to produce colored paper. Pigments are water insoluble compounds that are generally unbleachable. Dyes are water soluble, and are more commonly used than pigments to produce colored papers.
Dye molecules have two functional parts: the chromophore and the auxochrome. The chromophore is the portion that lends color to the dye. The auxochrome is the portion that anchors the dye to the cellulose fiber.
Chromophores are conjugated, that is, they contain a network of alternating single and double bonds. Light interacts with the conjugated network and is absorbed by electrons within the network. Wavelengths that are not absorbed by the network are reflected to produce the color in the fiber.
Conjugated networks vary in length, the length generally corresponding to a dye color. As the number of conjugated bonds in the network increases, the colors change from yellow to orange to red to violet to blue to brown to black and the networks become more susceptible to oxidation or free radical attack. Correspondingly, it can be difficult to decolorize yellow, orange, and red dyes due to their low degree of conjugation.
There are many different dye structures, but there are two main dye classes used by the paper industry: azo dyes and stilbene dyes. Stilbene dyes are less popular than azo dyes but are more stable to many oxidants, including chlorine.
An azo dye is a chemical compound containing at least one nitrogen-nitrogen double bond (azo functional group), which is subject to oxidative attack. A stilbene dye is an aromatic system containing a series of carbon--carbon double bonds which are more resistant to oxidative attack than are the nitrogen to nitrogen double bonds found in azo dyes.
To repulp and/or decolorize and effectively meet environmental requirements, it would be a breakthrough to find a suitable composition which would permit separation of the fiber without destruction of the cellulose, yet permit altering the chromphore to decolorize. Chlorine can repulp and/or decolorize but presents undesirable environmental problems. Persulfates can be used for repulping but are generally ineffective as decolorizing agents.